1. Field of the Invention
The present invention relates to an electrothermal printing apparatus, and more particularly, to a printing head for a resistive ribbon type printing apparatus. The ribbon used in such a printing apparatus comprises a flexible base insulating (or conductive) film, a layer of thermal transferable ink facing a paper, and an electrical resistive layer facing the printing head. When an electric current is made to flow through a portion of the resistive layer from the printing electrodes coming into contact with the resistive layer, Jonle heat is generated at the portion at which the current is flowing, to melt a portion of the ink layer, and the molten ink is transferred onto a paper to form a printed image.
2. Description of the Related Art
Such a resistive ribbon type printing apparatus and the printing head thereof have been proposed in, e.g., U.S. Pat. Nos. 3744611, 4350449, and 4456915. Also such printing heads are disclosed in, e.g., Japanese Unexamined Patent Publication (JUPP) Nos. 60-214972 and 60-214971.
According to JUPP No. 60-214972, a printing head is produced by printing a conductive paste including a hard metal such as W, Mo and Mn over the entire surface of a ceramic green sheet, sintering the printed green sheet, and selectively etching the sintered metal layer (metallized layer) to form a plurality of electrodes, by a photolithography process. In this case, for example, the printing head comprises a ceramic substrate made of magnesia and silicon dioxide and having a hardness of from 500 to 600 Hv, and tungsten electrodes having a hardness of about 700 Hv and a density of 3 electrodes per mm. When the printing head is operated, the substrate and electrodes come into contact with the ribbon simultaneously. Nevertheless, a conventional ceramic substrate has a relatively high hardness, and therefore, the electrodes should have a higher hardness, which leads to the problem of an insufficient formation of fine pattern electrodes, as it is difficult to selectively etch the hard metal (W) layer to form fine electrodes.
According to JUPP No. 60-214971, a printing head is produced by depositing a conductive layer over the entire surface of ceramic substrate by a vacuum evaporation or sputtering process, forming a plating layer on the conductive layer by an electroless plating process, and selectively etching the layers to form a plurality of electrodes by a photolithography process. In this case, for example, the ceramic substrate is made of magnesia and silicon dioxide and has a hardness of from 500 to 600 Hv, and the electrodes are made of Ni-W plating layer having a thickness of 10 .mu.m and a hardness of about 800 Hv. Since it is difficult to selectively etch such a hard alloy plating layer to form fine pattern electrodes, the obtained electrodes have an electrode density of 3 lines/mm.
As shown in FIG. 1, a conventional printing head 11 including the printing heads disclosed in JUPP Nos. 60-214972 and 60-214971 comprises a ceramic substrate 12 and a plurality of printing electrodes 13 formed on a top flat surface of the substrate 12. In this case, the printing head 11 chafes the resistive layer of the ribbon, and when an electric arc is occasionally generated between the electrodes and the resistive layer during the operation of the head 11, a portion (i.e., residue) of the resistive layer is removed and adheres to and accumulates on the top surface of the substrate 12 between the adjacent electrodes 13. This accumulation of this residue of the resistive layer will cause a short-circuiting between adjacent electrodes 13, and thus the quality of the printed image is greatly reduced.
To minimize this accumulation of residue of the resistive layer and prevent such short-circuiting, the present inventor proposed a printing head 15, as shown in FIG. 2, comprising a ceramic substrate 16 having a plurality of grooves having a segmentary cross section, which grooves are filled with electrodes 17 consisting of a lower plating layer 18 and a main plating layer 19. The head 15 is produced by etching the substrate 16 to form the grooves, forming the lower plating layer 18 in the grooves by an electroless (non-electrolytic) plating process and a selective etching process, depositing the main plating layer 19 by an electrolytic plating process, and grinding off any excess of the layer 19 to make the top surface of the head 15 flat. In this case, since the electrodes do not project above the surface of the substrate, the adhesion and accumulation of the residue of the resistive layer can be reduced. Nevertheless, when an electric arc is occasionally generated between the electrodes and the resistive layer of the ribbon, the arc will melt a portion of the resistive layer and the molten portion may adhere to an upper edge portion 20 of the substrate 16 as well as the electrodes 17. This adhered portion (i.e., resistive layer residue) forms an undesirable extension of the electrode and may cause short-circuiting between adjacent electrodes 17. Moreover, since the segment shaped grooves are formed by a photoetching process including the etching of the ceramic in a lateral direction (i.e., undercutting), the density of the formed electrodes is limited by the need for such undercutting.